Fine structural observations of the granulocytes involved in the tunic inflammatory-like reaction of Ciona intestinalis (Tunicata)

The conservation and diversity of ascidian cells and molecules involved in the inflammatory reaction: The Ciona robusta model

Ascidians are marine invertebrate chordates belonging to the earliest branch (Tunicata) in the chordate phylum, therefore, they are of interest for studying the evolution of immune systems. Due to the known genome, the non-colonial Ciona robusta, previously considered to be C. intestinalis type A, is a model species for the study of inflammatory response. The internal defense of ascidians mainly relies on hemocytes circulating in the hemolymph and pharynx. Hemocytes can be in vivo challenged by LPS injection and various granulocyte and vacuolated cell populations differentiated to produce and release inflammatory factors. Molecular biology and gene expression studies revealed complex defense mechanisms involving different inflammatory hemocytes. Furthermore, cloning procedures allowed sequence analyses and molecular studies disclose immune-related gene families including TOLL-like receptors, galectins, C-type lectins, collectins, interlectins, pentraxine-like, peroxinectins, complement factors-like, TNFα-like, IL-17-like, TGF-like, MIF-like. These genes are promptly upregulated by the inflammatory stimulus and show a time course of transcription similar to each other. Domains sequence similarity and phylogenetic relationships with the vertebrate counterparts are shedding some light on immune-related gene evolution. Selective bioassays as well as bioinformatic approaches have allowed the characterization of antimicrobial peptides and the identification of post transcriptional molecular mechanisms able of influencing dynamics of gene regulation are described. In synthesis, the purpose of this article is to further explore the topic of hemocyte and molecules related to internal defence of ascidians involved in the inflammatory reaction, as well as to discuss current and future study options through a detailed literature review.

Ciona robusta hemocyte populational dynamics and PO-dependent cytotoxic activity

Hemocyte populations from the ascidian Ciona robusta, separated through a Percoll discontinuous density gradient, are further characterized by May-Grünwald-Giemsa staining and a cytochemical reaction for phenoloxidase. Variability in cell density, acidophilic property and phenoloxidase activity suggest multiple hemocyte type populations, cell lineages and morphotypes that may be involved in distinct cellular responses. Therefore, unilocular refractile granulocytes, typical of this ascidian species, enriched in a fraction separated from the hemolymph show in vitro phenoloxidase-dependent cytotoxic activity against mammalian erythrocytes and a tumor cell lineage, in addition the properties listed above indicate relationships with vacuolated signet ring cells. Finally, bromo-deoxyuridine with, diamino-phenylindole fluorescent reaction and May-Grünwald-Giemsa staining show that in the hemolymph there are hyaline amoebocytes and granulocytes with potential proliferating activity. Present findings and reviewed images of previously reported inflammatory hemocytes in the tunic and pharynx allow us to speculate on theoretical outlines of hemocyte differentiation pathways.

Morphological characterization via light and electron microscopy of Atlantic jackknife clam (Ensis directus) hemocytes

The Atlantic jackknife clam, Ensis directus, is currently being researched as a potential species for aquaculture operations in Maine. The goal of this study was to describe the hemocytes of this species for the first time and provide a morphological classification scheme. We viewed hemocytes under light microscopy (using Hemacolor, neutral red, and Pappenheim's stains) as well as transmission electron microscopy (TEM). The 2 main types of hemocytes found were granulocytes and hyalinocytes (agranular cells). The granulocytes were subdivided into large and small granulocytes while the hyalinocytes were subdivided into large and small hyalinocytes. The large hemocytes had both a larger diameter and smaller nucleus to cell diameter ratio than their smaller counterparts. A rare cell type, the vesicular cell, was also observed and it possessed many vesicles but few or no granules. Using TEM, granulocytes were found to contain both electron-lucent and electron-dense granules of various sizes. These numerous granules were the only structures that took up the neutral red stain. Hyalinocytes had few of these granules relative to granulocytes. Large hyalinocytes had both various organelles and large vesicles in their abundant cytoplasm while small hyalinocytes had little room for organelles in their scant cytoplasm. Total hemocyte counts averaged 1.96×10(6) cells mL(-1) while differential hemocyte counts averaged 11% for small hyalinocytes, 12% for large hyalinocytes, 59% for small granulocytes, and 18% for large granulocytes. The results of this study provide a starting point for future studies on E. directus immune function.

Comparative study on the hemocytes of subtropical oysters Saccostrea kegaki (Torigoe & Inaba, 1981), Ostrea circumpicta (Pilsbry, 1904), and Hyotissa hyotis (Linnaeus, 1758) in Jeju Island, Korea: morphology and functional aspects

We first characterized the morphology and immune-related activities of hemocytes in the subtropical oysters Saccostrea kegaki, Ostrea circumpicta, and Hyotissa hyotis using light microscopy and flow cytometry. Hemocytes of these three oyster species were classified into three main types: 1) granulocytes containing numerous granules in the cytoplasm, 2) hyalinocytes with no or fewer granules, and 3) blast-like cells characterized by the smallest size and very thin cytoplasm. The percentage of each hemocyte population was similar in all species; hyalinocytes were the most abundant cell in the hemolymph accounting for more than 59%, followed by granulocytes (23-31%) and blast-like cells (3-5%). The size of granulocytes of S. kegaki was smaller (P < 0.05) than those of O. circumpicta and H. hyotis. Light microscopy also allowed the description of vacuolated cells characterized by large vacuoles in the cytoplasm. Flow cytometry analysis confirmed that the granulocytes of the three oyster species were the major hemocytes engaged in cellular defense with the largest lysosome content, and the most active phagocytosis activity and oxidative activity, as was previously reported in several marine bivalves. Phagocytic activity was the lowest in S. kegaki hemocytes, and PMA-stimulated oxidative activity was the lowest in H. hyotis hemocytes. Our results provide the basic information of hemocytes population of three subtropical oysters for further investigations associated with various environmental disease stresses.

Structure, biosynthesis and possible function of tunichromes and related compounds

Several species of ascidians (phylum Chordata, subphylum Urochordata) contain a group of oligopeptides called "tunichromes" in their blood cells. These peptides have been implicated in (a) metal chelation and accumulation/sequestration of vanadium or iron; (b) crosslinking of structural fibers in tunic formation, (c) wound healing and (d) defense reactions. However, their biosynthesis, metabolism, and biological function remain largely un-elucidated due to their extreme instability and high reactivity. Tunichromes and related compounds uniquely possess dehydrodopamine moieties, all originating from post-translational modification of peptidyl tyrosine. It is conceivable that the presence of such novel post-translationally modified groups provide attributes that are crucial for their biological roles. Therefore, we examined the chemistry and reactivity of tunichromes in light of the available knowledge of the biochemistry of simple monomeric dehydro-N-acyldopamine units. Based on the reactivity of such simple compounds, the potential biological activities of tunichromes are predicted. Their possible biosynthetic route from peptidyl tyrosine is critically evaluated to provide a better basis for unraveling their biological functions. Prevalence of dehydro-N-acyldopamine units in different tunichromes, some marine antibiotic compounds, insect cuticular sclerotizing precursors and some bioadhesive marine proteins may aid in the de novo design of unique biomaterials with potential antibiotic/adhesive properties.

Isolation and expression of a novel MBL-like collectin cDNA enhanced by LPS injection in the body wall of the ascidian Ciona intestinalis

Collectins are a family of calcium-dependent lectins that are characterized by their collagen-like domains. Considerable interest has been focused on this class of proteins because of their ability to interact with components of the complement system activating a cascade of events responsible for the activation of the innate immune system. A differential screening between LPS-challenged and naïve Ciona intestinalis has been performed allowing the isolation of a full length cDNA encoding for a 221 AA protein. In silico analysis has shown that this polypeptide displays protein domains with similarities to mannose-binding lectins. A phylogenetic analysis suggested that C. intestinalis MBL has evolved early as a prototype of vertebrate MBL. Real-time PCR assay demonstrated that this gene is strongly activated after LPS injection in the tunica. In situ hybridization performed in LPS-induced animals has shown that this gene is expressed in granular amoebocytes and large granules hemocytes in the inflamed body wall tissue. Finally, an antimicrobial activity of the C. intestinalis MBL has been demonstrated.

ciCD94-1, an ascidian multipurpose C-type lectin-like receptor expressed in Ciona intestinalis hemocytes and larval neural structures

C-type lectins play an important role in the immune system and are part of a large superfamily that includes C-type lectin-like domain (CTLD)-containing proteins. Divergent evolution, acting on the CTLD fold, has generated the Ca2+-dependent carbohydrate-binding lectins and molecules, as the lectin-like natural killer (NK) receptors that bind proteins, rather than sugars, in a Ca(2+)-independent manner. We have studied ciCD94-1, a CTLD-containing protein from the tunicate Ciona intestinalis, which is a homolog of the CD94 vertebrate receptor that is expressed on NK cells and modulates their cytotoxic activity by interacting with MHC class I molecules. ciCD94-1 shares structural features with the CTLD-containing molecules that recognize proteins, suggesting that it could be located along the evolutionary pathway leading to the NK receptors. ciCD94-1 was up-regulated in response to inflammation induced by lipopolysaccharide (LPS) acting on a blood cell type present in both the tunic and circulating blood. Furthermore, an anti-ciCD94-1 antibody specifically inhibited the phagocytic activity of these cells. ciCD94-1 was also expressed during development in the larva and in the early stages of metamorphosis in structures related to the nervous system, and loss of its function affected the correct differentiation of these territories. These findings suggest that ciCD94-1 has different roles in immunity and in development, thus strengthening the concept of gene co-option during evolution and of an evolutionary relationship between the nervous and the immune systems.

Inducible lectins with galectin properties and human IL1α epitopes opsonize yeast during the inflammatory response of the ascidian Ciona intestinalis

Studies on inducible ascidian lectins may shed light on the evolutionary emergence of cytokine functions. Here, we show that the levels of opsonins, with IL1alpha-epitopes, increase in Ciona intestinalis hemolymph as a response to an inflammatory stimulus and, in particular, to intratunic injection of lipopolysaccharide (LPS). The inflammatory agent promptly (within 4 h) enhances Ca(2+)-independent serum hemagglutinating and opsonizing activities, which are both inhibited by D-galactose and D-galactosides (alpha-lactose, N-acetyl-D-lactosamine, thio-digalactoside), suggesting that anti-rabbit erythrocyte lectins with galectin properties are involved as opsonins. Inducible galectin molecules contain interleukin-1alpha (IL1alpha) epitopes, and their activities are specifically inhibited by anti-human recombinant IL1alpha antibody. Analysis by SDS-polyacrylamide gel electrophoresis has revealed that the density of the bands of several serum proteins increases within 4 h after LPS injection, correlated with the enhanced serum activity. Moreover, Western blot patterns demonstrate that several serum proteins (59, 37, 30, 23, 15 kDa) cross-react with the antibody as early as 4 h post-injection. Although we have not been able to establish whether, in adition to galectins, various types of D-galactose-specific lectins are contained in the serum, we show, for the first time in invertebrates, that galectin molecules with opsonic properties can be enhanced in response to a non-specific inflammatory stimulus, and that their release can be further stimulated by LPS. Finally, we reveal that multiple galectins share human IL1alpha epitopes, probably because of steric configuration and the oligomerization process.

Rejection patterns in botryllid ascidian immunity: the first tier of allorecognition

Botryllid ascidians, a small but geographically widely distributed group of compound tunicates, are being used as a model system for the study of allorecognition. Botryllid ascidians possess a unique type of immunity. Pairs of colonies that meet through their extending ampullae either fuse to form a chimera or develop cytotoxic lesions at contact zones (rejection). This first tier of allorecognition is succeeded (in cases of fusion) by two additional tiers, not reviewed here (the colony resorption phenomenon and the phenomenon of somatic and germ cell parasitism). Fusion and rejection are controlled by a single highly polymorphic gene locus termed the fusibility/histocompatibility (Fu/HC) locus. One shared allele on the Fu/HC locus is enough for fusion. Rejecting colonies do not share any Fu/HC alleles. To date, 14 botryllid ascidians have been studied for their fusibility patterns; of these, the cosmopolitan species Botryllus schlosseri (Pallas, 1766) has emerged as the most studied taxon. This review summarizes studies revealing the various types of noncompatible responses that are expressed following the application of the "colony allorecognition assay" and the "cut surface assay". These include divergent alloresponses related to different populations of the same botryllid species, distinctive allorecognition sites, polymorphism and a repertoire of Fu/HC alleles, a state of low responsiveness as opposed to the expected immunological memory, the retreat growth phenomenon, and the irreversible nature of the rejection process. A detailed description of the accumulated knowledge on the effector cells (morula cells and macrophages), the humoral and cellular molecules (at the biochemical and molecular levels), and the prophenoloxidase system is given. Links between allogeneic responses and the evolutionary ecology of botryllid ascidians are revealed. Since tunicates occupy a key phylogenetic position in the origin of the vertebrates, the study of colony allorecognition in this group may shed light on self/nonself recognition elements in other multicellular organisms, including vertebrates.

CiC3-1a-mediated chemotaxis in the deuterostome invertebrate Ciona intestinalis (Urochordata)

Deuterostome invertebrates possess complement genes, and in limited instances complement-mediated functions have been reported in these organisms. However, the organization of the complement pathway(s), as well as the functions exerted by the cloned gene products, are largely unknown. To address the issue of the presence of an inflammatory pathway in ascidians, we expressed in Escherichia coli the fragment of Ciona intestinalis C3-1 corresponding to mammalian complement C3a (rCiC3-1a) and assessed its chemotactic activity on C. intestinalis hemocytes. We found that the migration of C. intestinalis hemocytes toward rCiC3-1a was dose dependent, peaking at 500 nM, and was specific for CiC3-1a, being inhibited by an anti-rCiC3-1a-specific Ab. As is true for mammalian C3a, the chemotactic activity of C. intestinalis C3-1a was localized to the C terminus, because a peptide representing the 18 C-terminal amino acids (CiC3-1a(59-76)) also promoted hemocyte chemotaxis. Furthermore, the CiC3-1a terminal Arg was not crucial for chemotactic activity, because the desArg peptide (CiC3-1a(59-75)) retained most of the directional hemocyte migration activity. The CiC3-1a-mediated chemotaxis was inhibited by pretreatment of cells with pertussis toxin, suggesting that the receptor molecule mediating the chemotactic effect is G(i) protein coupled. Immunohistochemical analysis with anti-rCiC3-1a-specific Ab and in situ hybridization experiments with a riboprobe corresponding to the 3'-terminal sequence of CiC3-1, performed on tunic sections of LPS-injected animals, showed that a majority of the infiltrating labeled hemocytes were granular amebocytes and compartment cells. Our findings indicate that CiC3-1a mediates chemotaxis of C. intestinalis hemocytes, thus suggesting an important role for this molecule in inflammatory processes.

Hemocyte migration during inflammatory-like reaction of Ciona intestinalis (Tunicata, ascidiacea)

Following an injuring factor such as the injection of erythrocytes into the tunic of Ciona intestinalis, an inflammatory-like response occurs and blood cells are massively involved in these mechanisms. Electron microscope observations illustrate the infiltration and migration of blood cells throughout the mantle epithelium into the tunic and show several prominent morphological changes.

Different types of response to foreign antigens by leech leukocytes

We used morphological and immunocytochemical approaches to characterize and to show the behavior of cells involved in leech inflammatory responses. Leeches were injected with bacterial lipopolysaccharide, fluoresceinated yeasts, sulfate spheres and ciliates (Protozoa). Shortly after injection, migrating cells appeared in the area of injection. The response of the cells occurred in relation to the injected micro or macro antigens. Each injection first provoked a migration of cells towards the non-self material. Afterwards, different responses (degranulation, phagocytosis, encapsulation, melanization) occurred. The migrating cells involved in these series of processes have a similar behavior and are characterized by CD markers of macrophages, NK cells and granulocytes, which are typical of many invertebrates and vertebrates.

Cell adhesion and the immune system: a case study using earthworms

In the earthworm's immune system, cell adhesion, which occurs by putative receptors on leukocytes, is essential after recognition of self vs. non-self. Confrontation with foreign antigens is a normal event in the environment, replete with microbial pathogens that pose a threat to survival. To better understand what happens when an effector cell first recognizes a foreign target followed by its adhesion to it, isolated leukocytes, in sufficient quantities to be subjected to various analyses, have been extremely beneficial. In vitro approaches when accompanied by biochemical, immunological, and molecular technologies, have opened up new vistas concerning the immune response of earthworms and other invertebrates. The most recent discovery includes the preliminary identification of cell differentiation (CD) markers that play vital roles in recognitive and adhesive events. Certain leukocyte effectors show characteristics of natural killer (NK) cells that may act differently depending upon their source, whether autogeneic, allogeneic, xenogeneic, or expressed under normal or varying environmental conditions including exposure to xenobiotics. At the level of earthworm evolution, there is apparently a dissociation of phagocytosis from the process of killing by NK-like effectors. There are at least three future challenges. First, it is essential to determine the precise nature of the CD markers with respect to their molecular structure. Second, once their molecular and biochemical characteristics have been defined, the role of these markers in cellular and humoral mechanisms must be clarified in order to define effector cell products and resulting immune responses. Third, there is a need to differentiate between the several lytic factors that have been found in earthworms with respect to molecular structure, and biochemical and functional characterization.

Sphingomyelin inhibition of Ciona intestinalis (Tunicata) cytotoxic hemocytes assayed against sheep erythrocytes

Hemocytes from the ascidian, Ciona intestinalis, are capable of lysing erythrocytes in vitro following cell membrane contact. With the aim of examining the mechanism of cytotoxicity, we performed inhibition experiments with lipid components of erythrocyte membranes. Cholesterol is not an inhibitor, whereas, among the phospholipids tested, (sphingomyelin, phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine) sphingomyelin inhibits the hemolytic activity of hemocytes. However, thin layer chromatography showed that sphingomyelinase activity was not contained in the chloroform-methanol extracts from hemocyte debris. The inhibition capacity of the components ceramide and phosphorylcholine suggests that the entire sphingomyelin molecule is involved in binding cytolysins. The lysin-lipid interactions probably cause changes in erythrocyte membrane permeability, leading to lysis.

Cytotoxic activity of Ciona intestinalis (Tunicata) hemocytes: properties of the in vitro reaction against erythrocyte targets

Hemocytes (effectors) of Ciona intestinalis showed a natural cytotoxic capacity (HCA) when assayed in vitro against erythrocytes (targets). Cytotoxic cells lysed, to a variable extent, rabbit (RE), human (A, B, O), guinea pig, and sheep (SE) erythrocytes. Hemocyte cytotoxic activity (HCA) assayed against SE is a calcium-dependent reaction, occurs rapidly (15-30 min), at 25-37 degrees C over a wide range of pH (5.4-8.0). Assays were carried out using: 1) the medium in which hemocytes were maintained, 2) the soluble portion of hemocyte lysates, and 3) debris prepared from hemocyte lysates. Results suggest that HCA is a cell-mediated process that requires effector-target cell contacts. Anti-SE (calcium-dependent) and anti-RE (calcium-independent) agglutinins were also found in the reaction medium, probably released by hemocytes as a consequence of the in vitro experiments. The occurrence of HCA was independent of any allogeneic reaction between mixed hemocytes. Various levels of cytotoxic activity reveal hemocyte specificity.